The invention relates to a cathode ray tube comprising an evacuated envelope in a neck portion of which there is situated an electron gun for generating three electron beams, and an electromagnetic deflection unit, outside the envelope, for deflecting the electron beams across a display screen.
The invention also relates to a method of manufacturing a cathode ray tube. Cathode ray tubes of the type mentioned in the opening paragraph are known. They are used, inter alia, in television receivers and computer monitors.
For a cathode ray tube, the quality of the image display is very important. Besides, the energy consumption and the depth dimension of the cathode ray tube are important too.
It is an object of the invention to provide a cathode ray tube in which an improved image display is possible, and in which the energy consumption can be reduced without the necessity of increasing the depth dimension of the cathode ray tube.
To achieve this, the cathode ray tube in accordance with the invention is characterized in that the neck portion includes a first part in which the electron gun is situated and a narrower second part located behind the electron gun, the largest distance between the centerlines of the electron beams upon leaving the gun ranging between 8 and 14 mm, and the deflection unit extending at least partly around the narrower part of the neck.
In a cathode ray tube, the image quality is determined, inter alia, by the largest distance between the electron beams, and the distance between the electron beams and the outer circumference of the neck portion where the deflection unit is situated.
In an electron gun there is a lens portion for focusing the electron beams. The quality of the lens is determined to a substantial degree by the size of the lens, which is determined by the shape and the size of the apertures in electrodes in the electron gun. The larger the distance between the electron beams, the larger the maximum lens diameter and the higher the lens quality is. The higher the lens quality, the better the electron beams can be focused on the display screen. However, as the distance between the beams increases, the distance between the electron beams in the deflection unit increases too, which has a negative effect on the accuracy with which the electron beams are deflected across the display screen. Particularly the convergence of the beams on the display screen is negatively influenced by an increase of the distance between the electron beams. In addition, the energy of the magnetic field necessary for deflecting the electron beams increases. Customarily, at each deflection stroke, this energy is transferred from the coil to a capacitive unit where it is stored. The transport losses involved are dissipated in the deflection unit which, as a result, is subject to an increase in temperature and heats up surrounding parts. An increase of the deflection frequency causes an increase in energy loss, since more energy transports between deflection unit and capacitive unit will take place per unit of time. This increase in temperature causes so-called thermal drift, which adversely affects the image displayed. Consequently, an increase or a reduction of the distance between the electron beams has opposite effects on the image and the deflection in a known cathode ray tube. In the cathode ray tube in accordance with the invention, the diameter of the neck exhibits a reduction behind the electron gun. As a result, the deflection unit can be provided closer to the beams, which results in an improved image quality and a reduced energy consumption. If the largest distance between the beams is less than 8 mm, then the quality of the lens will generally insufficiently meet present quality requirements. If the distance between the beams is more than 14 mm, then the energy consumption and the resultant thermal drift are generally too high.
Preferably, the narrower part has an outside diameter which is smaller than twice the distance between the electron beams. This enables a substantial reduction in energy consumption relative to the current designs to be achieved.
In electron guns of the so-called in-line type, in which three co-planar electron beams are generated, the largest distance between electron beams is formed by the distance between the outermost electron beams, i.e. the so-called red and blue electron beams. In electron guns of the so-called Delta type, the largest distance between electron beams is formed by the distance between two random electron beams.
Preferably, the electron gun is provided with a centring cup having a length below 5 mm, and preferably below 3 mm. In a known electron gun, the length of the centring cup is approximately 7-8 mm. By reducing the length of the centring unit, the electron gun can be arranged closer to the deflection unit. As a result, the depth dimension of the cathode ray tube is reduced.
Preferably, the cathode ray tube is provided with a deflection unit including deflection coils and a deflection-coil support which, on one side, exhibits a neck-shaped aperture, said deflection coil support having means for reversibly widening the neck-shaped aperture, such that the deflection coil support constitutes a coherent whole.
By virtue thereof, the deflection coil support can be readily provided on the envelope of the cathode ray tube.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.